Oil and gas production will in most cases have to be stopped when the water production from a well becomes excessive. The time of water breakthrough will vary from one zone to the next, and will also depend on the measured depth of the zone due to flow pressure drop. If a zone that mainly has an inflow of water is choked, the production from zones producing mainly oil may be increased. As a result, systems have been produced in recent years which comprise valves and adjustable nozzles controlled from the surface. These are technically complex systems that require a great amount of downhole equipment, and which have so far shown poor reliability. Also, the potential for using more than 4–5 valves in each well is limited. In addition, the flow area of the production tubing is small, limiting the production.
As a simple alternative to this, a nozzle or ducting system has been developed in which the production is restricted regardless of whether the inflow consists of oil or water. Examples of this are seen in U.S. Pat. Nos. 6,112,815 and 5,435,393. The arrangements according to these documents may counter frictional effects caused by the flow of fluid flowing through the production tubing, but will not regulate the pressure drop across the system on the basis of the water cut in the wellstream. According to these patents, the produced fluids flow through a fixed flow restriction such as a capillary tube or nozzle, before flowing into the tubing. These capillary tube devices have typically been arranged around the production tubing as a helical thread where the fluid flows in the grooves of the thread.
U.S. Pat. No. 5,333,684 discusses a tool for drawing gas out of a well without simultaneously producing water. The tool is equipped with spherical, stacked controlled buoyancy elements, where the density of the buoyancy elements is lower than that of water. Upon outflow of water from the well, the elements ascend and close an opening, preventing water from flowing out of the well.